1. Field of the Invention
The present invention relates to laser amplifiers, and particularly to laser amplifiers generating high peak power and high energy per pulse output beams, e.g., for generating X-rays used in the process of X-ray lithography for manufacturing of integrated circuits.
2. Description of Related Art
High power laser amplifiers have a wide variety of applications. One important example is in the generation of X-rays used for X-ray lithography in the manufacture of integrated circuits. To generate X-rays, 1-20 nanosecond pulses of infrared radiation of about 20 joules per pulse with peak powers over a gigawatt are needed. Also, these pulses must be generated from five to ten times per second to achieve sufficient performance for cost effective production of integrated circuits. These high power infrared pulses are directed onto a tape impregnated with an iron oxide, which generates an X-ray in response to the stimulation of the infrared pulse. The X-rays are then used to illuminate resist coated wafers in the X-ray lithography process producing integrated circuits.
The design of laser amplifiers which can achieve these performance goals has been limited in the prior art by a variety of factors related to the tolerance of optical components in the amplifier to pulses of high energy laser light.
For instance, in one large class of amplifier designs, known as regenerative amplifiers, multiple passes through a single gain medium, or plural gain media, are used for efficient extraction of energy. In these regenerative amplifiers, an optical path is defined around which an input pulse transits a number of times.
The efficient extraction of energy from the gain medium is limited, however, by losses in optical components in the path, such as electro-optic switches, polarizers, and the like. For amplifiers which involve numerous transits of the optical path, a small loss in a single component can decrease the gain to loss ratio of the amplifier significantly.
Furthermore, the optical components typically have peak power damage thresholds. Perturbations or diffraction in the beam as the beam transits the optical path can cause the beam to exceed these peak power damage thresholds. This results in damage to the optics and loss in efficiency in the regenerative amplifier.
Another limitation in these multipass systems resides in average power thresholds of optical components in the optical path. For systems which involve a number of transits of the optical path and repetitive pulsed operation, the average power dissipated in a given optical element can be quite high.
A representative regenerative amplifier design of the prior art is shown in FIG. 1, which is a schematic diagram of a high average power amplifier described by Summers, et al., "Design Performance of a High Average Power Zig-Zag Slab Laser", Optical Society of America, 1989 Annual Meeting in Orlando, Fla.
The amplifier design of FIG. 1 includes a first polarizer 10, a first electro-optic switch or Pockels cell 11, a second polarizer 12, a first mirror 13, a second Pockels cell 14, a third polarizer 15, a zig-zag amplifier 16, a second mirror 17, a third mirror 18, an anamorphic vacuum relay (telescope) 19, and a fourth mirror 20. Also, a lense 21, fifth mirror 22, lense 23, and phase conjugator 24 are included in the amplifier system.
In operation, an input pulse is supplied incident on the first polarizer 10, and having a polarization which is reflected by the polarizer 10. This input beam passes through the first Pockels cell 11 without rotation, and is reflected by the second polarizer 12 into a ring shaped optical path. From the second polarizer 12, the pulse proceeds to mirror 13 and Pockels cell 14, where it is rotated to a polarization which is transmitted by the third polarizer 15. It then proceeds through the zig-zag amplifier 16, mirror 17, mirror 18, telescope 19, mirror 20, through second polarizer 12, to mirror 13, and through the Pockels cell 14 without rotation. Thus, the pulse is captured within the ring for a number of passes to achieve high gain. After one or more passes through the amplifier 16, the Pockels cell 14 causes the pulse to rotate so that it is reflected by third polarizer 15 into the phase conjugation leg of the amplifier. When it returns from the phase conjugator 24, it is again reflected by third polarizer 15 and supplied through Pockels cell 14 where it is rotated back to the polarization transmitted by the polarizers. It is then captured within the ring proceeding in the opposite direction for one or more passes through the amplifier. To couple the pulse out of the ring, the Pockels cell 14 rotates the pulse proceeding from third polarizer 15 toward mirror 13 so that it is reflected by second polarizer 12 through Pockels cell 11. Pockels cell 11 rotates the pulse so that it is transmitted by first polarizer 10 and supplied as an output beam.
This amplifier design demonstrates many of the limitations of the prior art. As can be seen, each pass through the amplifier 16 in which gain is achieved also involves a pass through a number of elements which can cause significant loss, including the telescope 19, the polarizers 12 and 15, and the electro-optic switch, implemented by the second Pockels cell 14.
Also, each of these elements is sensitive to perturbations in the beam. To limit the damage caused by perturbations, the relay telescope 19 relays an image near the amplifier 16 back onto itself. However, mirrors of the relay telescope 19 are far from the image plane, and thus diffraction of the beam in propagating from the plane results in intensity spiking and limited power.
Because of the above listed limitations, the amplifier design of FIG. 1 is impractical to use for producing the energy per pulse and peak power required in production of integrated circuits using X-rays, and for a variety of other applications. Accordingly, it is desirable to provide an amplifier design overcoming these prior art limitations.